Method for the production of butadiene from n-butane

ABSTRACT

The invention relates to a process for preparing butadiene from n-butane comprising the steps (A) providing an n-butane-containing feed gas stream, (B) feeding the n-butane-containing feed gas stream into a first dehydrogenation zone and nonoxidatively catalytically dehydrogenating n-butane to 1-butene, 2-butene and optionally butadiene to obtain a first product gas stream comprising n-butane, 1-butene and 2-butene, with or without butadiene and secondary components, (C) feeding the first product gas stream comprising n-butane, 1-butene and 2-butene, with or without butadiene and secondary components, into a second dehydrogenation zone and oxidatively dehydrogenating 1-butene and 2-butene to butadiene to give a second product gas stream comprising butadiene, n-butane and steam, with or without secondary components, (D) recovering butadiene from the second product gas stream.

Butadiene is prepared predominantly by thermal cleavage (cracking) ofsaturated hydrocarbons, customarily starting from naphtha as the rawmaterial. Cracking of naphtha results in a hydrocarbon mixture ofmethane, ethane, ethene, acetylene, propane, propene, propyne, allene,butenes, butadiene, butynes, methylallene, C₅ and higher hydrocarbons.Acetylenically unsaturated hydrocarbons in the cracking gas such asacetylene, propyne, 1-butyne, 2-butyne, butenyne and diacetylene mayinterfere, for example, in a subsequent dimerization of butadiene in aDiels-Alder reaction to vinylcyclohexane, since even traces of thesecompounds can poison the copper dimerization catalyst. Butynes andallenes likewise react with butadiene in a Diels-Alder reaction and leadto by-product formation. Triply unsaturated C₄ hydrocarbons aregenerally also troublesome in other uses of butadiene.

The butynes in particular, which can only be removed distillatively orextractively from butadiene with great difficulty, present problems. Itis therefore necessary when using butadiene from crackers to precede thebutadiene dimerization with a hydrogenation stage in which the butynesare selectively partially hydrogenated to the corresponding butenes.

A further disadvantage is that when cracking naphtha or otherhydrocarbon mixtures, a complex hydrocarbon mixture is obtained. Forinstance, when butadiene is obtained in the cracking process, relativelylarge amounts of ethene or propene are inevitably obtained ascoproducts.

Alternatively, butadiene can be prepared starting from n-butane bycatalytic dehydrogenation. However, a disadvantage of this process isthe low butadiene yield, since the catalytic dehydrogenation of n-butaneresults predominantly in 1-butene and 2-butene.

It is an object of the present invention to provide a process forpreparing butadiene from n-butane which does not have the disadvantagesof the prior art and allows high butadiene yields to be obtained.

We have found that this object is achieved by a process for preparingbutadiene from n-butane comprising the steps of

-   (A) providing an n-butane-containing feed gas stream,-   (B) feeding the n-butane-containing feed gas stream into a first    dehydrogenation zone and nonoxidatively catalytically    dehydrogenating n-butane to 1-butene, 2-butene and optionally    butadiene to obtain a first product gas stream comprising n-butane,    1-butene and 2-butene, with or without butadiene and secondary    components,-   (C) feeding the first product gas stream comprising n-butane,    1-butene and 2-butene, with or without butadiene and secondary    components, into a second dehydrogenation zone and oxidatively    dehydrogenating 1-butene and 2-butene to butadiene to give a second    product gas stream comprising butadiene, n-butane and steam, with or    without secondary components,-   (D) recovering butadiene from the second product gas stream.

In a first process part A, an n-butane-containing feed gas stream isprovided. Customarily, the raw material is an n-butane-rich gas mixturesuch as liquefied petroleum gas (LPG).

LPG substantially comprises C₂-C₅-hydrocarbons. The composition of LPGmay vary widely. Advantageously, the LPG used comprises at least 10% byweight of butanes.

In one variant of the process according to the invention, the provisionof the n-butane-containing dehydrogenation feed gas stream comprises thesteps of

-   (A1) providing a liquefied petroleum gas (LPG) stream,-   (A2) removing propane and optionally methane, ethane and pentanes    from the LPG stream to obtain a butane- (n-butane- and isobutane-)    containing stream,-   (A3) removing isobutane from the butane-containing stream to obtain    the n-butane-containing feed gas stream and optionally isomerizing    the removed isobutane to an n-butane/isobutane mixture and recycling    the n-butane/isobutane mixture into the isobutane removal.

Propane and any methane, ethane and pentanes are removed in one or morecustomary rectification columns. For example, low boilers (methane,ethane, propane) can be removed overhead in a first column and highboilers (pentanes) removed at the column bottom in a second column. Astream comprising butanes (n-butane and isobutane) is obtained fromwhich isobutane is removed, for example in a customary rectificationcolumn. The remaining n-butane-containing stream is used as the feed gasstream for the subsequent butane dehydrogenation.

Preference is given to subjecting the removed isobutane stream toisomerization. To this end, the isobutane-containing stream is fed intoan isomerization reactor. The isomerization of isobutane to n-butane canbe carried out as described in GB-A 2 018 815.

An n-butane/isobutane mixture is obtained which is fed into then-butane/isobutane separating column.

In a process part (B), the n-butane-containing feed gas stream is fedinto a first dehydrogenation zone and subjected to a nonoxidativecatalytic dehydrogenation. n-Butane is partially dehydrogenated in adehydrogenation reactor over a dehydrogenating catalyst to 1-butene and2-butene, and small amounts of butadiene may also be formed. Inaddition, hydrogen and small amounts of methane, ethane, ethene, propaneand propene are formed. Depending on the dehydrogenation method, carbonoxides (CO, CO₂), water and nitrogen may also be present in the productgas mixture of the nonoxidative catalytic n-butane dehydrogenation. Inaddition, unconverted n-butane is present in the product gas mixture.

The nonoxidative catalytic n-butane dehydrogenation may be carried outwith or without oxygen-containing gas as a cofeed.

A feature of the nonoxidative method compared to an oxidative method isthe presence of hydrogen in the effluent gas. In the oxidativedehydrogenation, no substantial amounts of free hydrogen are formed.

In principle, the nonoxidative catalytic n-butane dehydrogenation may becarried out in all reactor types and methods known from the prior art. Acomparatively comprehensive description of dehydrogenation processessuitable according to the invention may also be found in “Catalytica®Studies Division, Oxidative Dehydrogenation and AlternativeDehydrogenation Processes” (Study Number 4192 OD, 1993, 430 FergusonDrive, Mountain View, Calif. 94043-5272, USA).

A suitable reactor form is a fixed bed tubular or tube bundle reactor.In these reactors, the catalyst (dehydrogenation catalyst and, whenworking with oxygen as the cofeed, optionally a special oxidationcatalyst) is disposed as a fixed bed in a reaction tube or in a bundleof reaction tubes. The reaction tubes are customarily heated indirectlyby the combustion of a gas, for example a hydrocarbon such as methane,in the space surrounding the reaction tubes. It is favorable to applythis indirect form of heating only to about the first 20 to 30% of thelength of the fixed bed and to heat the remaining bed length to therequired reaction temperature by the radiant heat released in the courseof indirect heating. Customary reaction tube internal diameters are fromabout 10 to 15 cm. A typical dehydrogenation tube bundle reactorcomprises from about 300 to 1 000 reaction tubes. The internaltemperature in the reaction tubes is customarily in the range from 300to 1 200° C., preferably in the range from 500 to 1 000° C. The workingpressure is customarily from 0.5 to 8 bar, frequently from 1 to 2 bar,when a small steam dilution is used (similar to the Linde process forpropane dehydrogenation), or else from 3 to 8 bar when using a highsteam dilution (similar to the steam active reforming process (STARprocess) for dehydrogenating propane or butane of Phillips PetroleumCo., see U.S. Pat. No. 4,902,849, U.S. Pat. No. 4,996,387 and U.S. Pat.No. 5,389,342). Typical gas hourly space velocities (GSHV) are from 500to 2 000 ⁻¹, based on the hydrocarbon used. The catalyst geometry may,for example, be spherical or cylindrical (hollow or solid).

The nonoxidative catalytic n-butane dehydrogenation may also be carriedout under heterogeneous catalysis in a fluidized bed, as described inChem. Eng. Sci. 1992 b, 47 (9-11) 2313. Advantageously, two fluidizedbeds are operated in parallel, of which one is generally in the processof regeneration. The working pressure is typically from 1 to 2 bar, thedehydrogenation temperature generally from 550 to 600° C. The heatrequired for the dehydrogenation is introduced into the reaction systemby preheating the dehydrogenation catalyst to the reaction temperature.The admixing of an oxygen-containing cofeed allows the preheater to bedispensed with and the required heat can be generated directly in thereactor system by combustion of hydrogen in the presence of oxygen.Optionally, a hydrogen-containing cofeed may additionally be admixed.

The nonoxidative catalytic n-butane dehydrogenation may be carried outin a tray reactor with or without oxygen-containing gas as cofeed. Thisreactor comprises one or more successive catalyst beds. The number ofcatalyst beds may be from 1 to 20, advantageously from 1 to 6,preferably from 1 to 4 and in particular from 1 to 3. The catalyst bedsare preferably flowed through radially or axially by the reaction gas.In general, such a tray reactor is operated using a fixed catalyst bed.In the simplest case, the fixed catalyst beds are disposed axially in ashaft furnace reactor or in the annular gaps of concentric cylindricalgrids. A shaft furnace reactor corresponds to one tray. Carrying out thedehydrogenation in a single shaft furnace reactor corresponds to apreferred embodiment, where the oxygen-containing cofeed may be used. Ina further preferred embodiment, the dehydrogenation is carried out in atray reactor having three catalyst beds. In a method withoutoxygen-containing gas as cofeed, the reaction gas mixture is subjectedto a degree of heating in the tray reactor on its way from one catalystbed to the next catalyst bed, for example by passing it over heatexchanger plates heated by hot gases or by passing it through tubesheated by hot combustion gases.

In a preferred embodiment of the process according to the invention, thenonoxidative catalytic n-butane dehydrogenation is carried outautothermally. To this end, the reaction gas mixture of the n-butanedehydrogenation is additionally admixed with oxygen in at least onereaction zone and the hydrogen and/or hydrocarbon present in thereaction gas mixture is at least partially combusted which directlygenerates in the reaction gas mixture at least a portion of the heatrequired for dehydrogenation in the at least one reaction zone.

In general, the amount of oxygen-containing gas added to the reactiongas mixture is chosen in such a manner that the amount of heat requiredfor the dehydrogenation of n-butane is generated by the combustion ofthe hydrogen present in the reaction gas mixture and any hydrocarbonspresent in the reaction gas mixture and/or carbon present in the form ofcoke. In general, the total amount of oxygen fed in, based on the totalamount of butane, is from 0.001 to 0.5 mol/mol, preferably from 0.005 to0.2 mol/mol, more preferably from 0.05 to 0.2 mol/mol. Oxygen may beused either as pure oxygen or as an oxygen-containing gas in the mixturewith inert gases, for example in the form of air. The inert gases andthe gases resulting from the combustion generally provide additionaldilution and therefore support the heterogeneously catalyzeddehydrogenation.

The hydrogen combusted to generate heat is the hydrogen formed in thecatalytic n-butane dehydrogenation and also any hydrogen additionallyadded to the reaction gas mixture as hydrogen-containing gas. Thequantity of hydrogen present should preferably be such that the H₂/O₂molar ratio in the reaction gas mixture immediately after the oxygen isfed in is from 1 to 10 mol/mol, preferably from 2 to 5 mol/mol. Inmultistage reactors, this applies to every intermediate feed ofoxygen-containing and any hydrogen-containing gas.

The hydrogen is combusted catalytically. The dehydrogenation catalystused generally also catalyzes the combustion of the hydrocarbons and ofhydrogen with oxygen, so that in principle no additional specializedoxygenation catalyst is required. In one embodiment, operation iseffected in the presence of one or more oxidation catalysts whichselectively catalyze the combustion of hydrogen to oxygen in thepresence of hydrocarbons. The combustion of these hydrocarbons withoxygen to give CO, CO₂ and water therefore proceeds only to a minorextent. The dehydrogenation catalyst and the oxidation catalyst arepreferably present in different reaction zones.

When the reaction is carried out in more than one stage, the oxidationcatalyst may be present only in one, in more than one or in all reactionzones.

Preference is given to disposing the catalyst which selectivelycatalyzes the oxidation of hydrogen at the points where there are higheroxygen partial pressures than at other points in the reactor, inparticular near the feed point for the oxygen-containing gas. Theoxygen-containing gas and/or hydrogen-containing gas may be fed in atone or more points in the reactor.

In one embodiment of the process according to the invention, there isintermediate feeding of oxygen-containing gas and of hydrogen-containinggas upstream of each tray of a tray reactor. In a further embodiment ofthe process according to the invention, oxygen-containing gas andhydrogen-containing gas are fed in upstream of each tray except thefirst tray. In one embodiment, a layer of a specialized oxygenationcatalyst is present downstream of every feed point, followed by a layerof the dehydrogenation catalyst. In a further embodiment, no specializedoxidation catalyst is present. The dehydrogenation temperature isgenerally from 400 to 1 100° C., the pressure in the last catalyst bedof the tray reactor is generally from 0.2 to 5 bar, preferably from 1 to3 bar. The GSHV is generally from 500 to 2 000 h⁻¹, and in a high-loadoperation, even up to 100 000 h⁻¹, preferably from 4 000 to 16 000 h⁻¹.

A preferred catalyst which selectively catalyzes the combustion ofhydrogen comprises oxides and/or phosphates selected from the groupconsisting of oxides and/or phosphates or germanium, tin, lead, arsenic,antimony and bismuth. A further preferred catalyst which catalyzes thecombustion of hydrogen comprises a noble metal of transition group VIIIand/or I of the periodic table.

The dehydrogenation catalysts used generally comprise a support and anactive composition. The support generally consists of a heat-resistantoxide or mixed oxide. The dehydrogenation catalysts preferably comprisea metal oxide selected from the group consisting of zirconium oxide,zinc oxide, aluminum oxide, silicon dioxide, titanium dioxide, magnesiumoxide, lanthanum oxide, cerium oxide and mixtures thereof, as support.The mixtures may be physical mixtures or else chemical mixed phases ofmagnesium aluminum oxide or zinc aluminum oxide mixed oxides. Preferredsupports are zirconium dioxide and/or silicon dioxide, and particularpreference is given to mixtures of zirconium dioxide and silicondioxide.

The active composition of the dehydrogenation catalysts generallycomprises one or more metals of transition group VIII of the periodictable, preferably platinum and/or palladium, more preferably platinum.Furthermore, the dehydrogenation catalysts may comprise one or moreelements of main group I and/or II of the period table, preferablypotassium and/or cesium. The dehydrogenation catalysts may furthercomprise one or more elements of transition group III of the periodtable including the lanthanides and actinides, preferably lanthanumand/or cerium. Finally, the dehydrogenation catalysts may comprise oneor more elements of main group III and/or IV of the periodic table,preferably one or more elements selected from the group consisting ofboron, gallium, silicon, germanium, tin and lead, more preferably tin.

In a preferred embodiment, the dehydrogenation catalyst comprises atleast one element of transition group VIII, at least one element of maingroup I and/or II, at least one element of main group III and/or IV andat least one element of transition group III including the lanthanidesand actinides, of the periodic table.

For example, all dehydrogenation catalysts which are disclosed by WO99/46039, U.S. Pat. No. 4,788,371, EP-A 705 136, WO 99/29420, U.S. Pat.No. 5,220,091, U.S. Pat. No. 5,430,220, U.S. Pat. No. 5,877,369, EP 0117 146, DE-A 199 37 106, DE-A 199 37 105 and DE-A 199 37 107 may beused according to the invention. Particularly preferred catalysts forthe above-described variants of autothermal n-butane dehydrogenation arethe catalysts according to examples 1, 2, 3 and 4 of DE-A 199 37 107.

Preference is given to carrying out the n-butane dehydrogenation in thepresence of steam. The added steam serves as a heat carrier and supportsthe gasification of organic deposits on the catalysts, which counteractscarbonization of the catalysts and increases the on stream time of thecatalysts. The organic deposits are converted to carbon monoxide, carbondioxide and possibly water.

The dehydrogenation catalysts may be regenerated in a manner known perse. For instance, steam may be added to the reaction mixture or anoxygen-containing gas may be passed from time to time over the catalystbed at elevated temperature and the deposited carbon burnt off. Dilutionwith steam shifts the equilibrium toward the products ofdehydrogenation. After the regeneration with steam, the catalyst isoptionally reduced with a hydrogen-containing gas.

The n-butane dehydrogenation provides a gas mixture which, in additionto butadiene, 1-butene, 2-butene and unconverted n-butane, comprisessecondary components. Customary secondary components include hydrogen,steam, nitrogen, CO and CO₂, methane, ethane, ethene, propane andpropene. The composition of the gas mixture leaving the firstdehydrogenation zone may be highly variable depending on thedehydrogenation method. For instance, in the preferred autothermaldehydrogenation with feeding in of oxygen and in addition of hydrogen,the product gas mixture comprises a comparatively high content of steamand carbon oxides. In methods without feeding in of oxygen, the productgas mixture of the nonoxidative dehydrogenation has a comparatively highhydrogen content.

The product gas stream of the nonoxidative autothermal n-butanedehydrogenation typically comprises from 0.1 to 15% by volume ofbutadiene, from 1 to 15% by volume of 1-butene, from 1 to 20% by volumeof 2-butene, from 20 to 70% by volume of n-butane, from 5 to 70% byvolume of steam, from 0 to 5% by volume of low-boiling hydrocarbons(methane, ethane, ethene, propane and propene), from 0 to 30% by volumeof hydrogen, from 0 to 30% by volume of nitrogen and from 0 to 5% byvolume of carbon oxides.

According to the invention, the nonoxidative catalytic dehydrogenationis followed by an oxidative dehydrogenation (oxydehydrogenation) asprocess part C.

In principle, this may be carried out in all reactor types and methodsknown from the prior art, for example in a fluidized bed, in a trayfurnace or a fixed bed tubular or tube bundle reactor. Preference isgiven to using the latter in the process according to the invention. Tocarry out the oxydehydrogenation, a gas mixture is required which has amolar oxygen: n-butene ratio of at least 0.5. Preference is given to anoxygen: n-butene ratio of from 0.55 to 50. To adjust this ratio, theproduct gas mixture which generally results from the catalyticdehydrogenation is mixed with oxygen or an oxygen-containing gas, forexample air. The oxygen-containing gas mixture obtained is then fed tothe oxydehydrogenation.

The catalysts which are particularly suitable for the oxydehydrogenationof the n-butenes to 1,3-butadiene are generally based on an Mo—Bi—Omultimetal oxide system which generally additionally comprises iron. Ingeneral, the catalyst system also comprises additional components fromgroups 1 to 15 of the periodic table, for example potassium, magnesium,zirconium, chromium, nickel, cobalt, cadmium, tin, lead, germanium,lanthanum, manganese, tungsten, phosphorus, cerium, aluminum or silicon.

Useful catalysts and their preparation are described, for example, inU.S. Pat. No. 4,423,281 (Mo₁₂BiNi₈Pb_(0.5)Cr₃K_(0.2)O_(x) andMo₁₂Bi_(b)Ni₇Al₃Cr_(0.5)K_(0.5)O_(x)), U.S. Pat. No. 4,336,409(Mo₁₂BiNi₆Cd₂Cr₃P_(0.5)O_(x)), DE-A 26 00 128(Mo₁₂BiNi_(0.5)Cr₃P_(0.5)Mg_(7.5)K_(0.1)O_(x)+SiO₂) and DE-A 24 40 329(Mo₁₂BiCo_(4.5)Ni_(2.5)Cr₃P_(0.5)K_(0.1)O_(x)), which are explicitlyincorporated herein by way of reference.

The stoichiometry of the active composition of a variety of multimetaloxide catalysts suitable for the oxydehydrogenation of the n-butenes to1,3-butadiene can be subsumed under the general formula (I)Mo₁₂Bi_(a)Fe_(b)Co_(c)Ni_(d)Cr_(e)X¹ _(f)K_(g)O_(x)  (I)where the variables are defined as follows:

-   X¹=W, Sn, Mn, La, Ce, Ge, Ti, Zr, Hf, Nb, P, Si, Sb, Al, Cd and/or    Mg;-   a=from 0.5 to 5, preferably from 0.5 to 2;-   b=from 0 to 5, preferably from 2 to 4;-   c=from 0 to 10, preferably from 3 to 10;-   d=from 0 to 10;-   e=from 0 to 10, preferably from 0.1 to 4;-   f=from 0 to 5, preferably from 0.1 to 2;-   g=from 0 to 2, preferably from 0.01 to 1; and-   x=a number which is determined by the valency and frequency of the    elements in (I) other than oxygen.

In the process according to the invention, preference is given to usingan Mo—Bi—Fe—O multimetal oxide system for the oxydehydrogenation, andparticular preference is given to a Mo—Bi—Fe—Cr—O or Mo—Bi—Fe—Zr—O metaloxide system. Preferred systems are described, for example, in U.S. Pat.No. 4,547,615 (Mo₁₂BiFe_(0.1)Ni₈ZrCr₃K_(0.2)O_(x) andMo₁₂BiFe_(0.1)Ni₈AlCr₃K_(0.2)O_(x)), U.S. Pat. No. 4,424,141(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)P_(0.5)K_(0.1)O_(x)+SiO₂), DE-A 25 30 959(MO₁₂BiFe₃CO_(4.5)Ni_(2.5)Cr_(0.5)K_(0.1)O_(x),Mo_(13.75)BiFe₃Co_(4.5)Ni_(2.5)Ge_(0.5)K_(0.8)O_(x),Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Mn_(0.5)K_(0.1)Ox andMo₁₂BiFe₃Co_(4.5)Ni_(2.5)La_(0.5)K_(0.1)O_(x)), U.S. Pat. No. 3,911,039(MO₁₂BiFe₃CO_(4.5)Ni_(2.5)Sn_(0.5)K_(0.1)O_(x)), DE-A 25 30 959 and DE-A24 47 825 (Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)W_(0.5)K_(0.1)O_(x)). Thepreparation and characterization of the catalysts mentioned aredescribed comprehensively in the documents cited to which reference ishereby explicitly made.

The oxydehydrogenation catalyst is generally used as shaped bodieshaving an average size of over 2 mm. Owing to the pressure drop to beobserved when performing the process, smaller shaped bodies aregenerally unsuitable. Examples of useful shaped bodies include tablets,cylinders, hollow cylinders, rings, spheres, strands, wagon wheels orextrudates. Special shapes, for example “trilobes” and “tristars” (seeEP-A-0 593 646) or shaped bodies having at least one notch on theexterior (see U.S. Pat. No. 5,168,090) are likewise possible.

In general, the catalysts used may be used as an unsupported catalyst.In this case, the entire shaped catalyst body consists of the activecomposition, including any auxiliary, such as graphite or pore formerand also further components. In particular, it has proven advantageousto use the Mo—Bi—Fe—O catalyst preferably used for theoxydehydrogenation of n-butenes to butadiene as an unsupported catalyst.Furthermore, it is possible to apply the active compositions of thecatalysts to a support, for example an inorganic, oxidic shaped body.Such catalysts are generally referred to as coated catalysts.

The oxydehydrogenation of the n-butenes to butadiene is generallycarried out at a temperature of from 220 to 490° C. and preferably from250 to 450° C. For practical reasons, a reactor entrance pressure isgenerally chosen which is sufficient to overcome the flow resistances inthe plant and the subsequent workup. This reactor entrance pressure isgenerally from 0.005 to 1 MPa above atmospheric pressure, preferablyfrom 0.01 to 0.5 MPa above atmospheric pressure. By its nature, the gaspressure applied in the entrance region of the reactor substantiallyfalls over the entire catalyst bed and inert fractions.

The coupling of the nonoxidative catalytic, preferably autothermaldehydrogenation with the oxidative dehydrogenation of the n-butenesformed provides a very much higher yield of butadiene based on n-butaneused. The nonoxidative dehydrogenation can also be operated in a gentlermanner. Comparable yields would only be achievable with an exclusivelynonoxidative dehydrogenation at the cost of distinctly reducedselectivities.

In addition to butadiene and unconverted n-butane, the second productgas stream leaving the oxydehydrogenation comprises steam. As secondarycomponents it generally comprises carbon monoxide, carbon dioxide,oxygen, nitrogen, methane, ethane, ethene, propane and propene, with orwithout hydrogen and also oxygen-containing hydrocarbons, known asoxygenates. In general, it only comprises very small proportions of1-butene and 2-butene.

For example, the product gas stream leaving the oxydehydrogenation maycomprise from 1 to 20% by volume of butadiene, from 0 to 1% by volume of1-butene, from 0 to 1% by volume of 2-butene, from 0 to 50% by volume ofbutane, from 2 to 50% by volume of steam, from 0 to 5% by volume oflow-boiling hydrocarbons (methane, ethane, ethene, propane and propene),from 0 to 20% by volume of hydrogen, from 0 to 90% by volume ofnitrogen, from 0 to 5% by volume of carbon oxides and from 0 to 3% byvolume of oxygenates.

Butadiene is recovered in a process part D from the second product gasstream obtained in the oxydehydrogenation.

The recovery of butadiene from the second product gas stream maycomprise the following steps:

-   (D1) cooling the product gas stream with water to condense out steam    and any high-boiling organic secondary components;-   (D2) removing the low-boiling secondary components contained in the    second product gas stream which are selected from the group    consisting of hydrogen, carbon monoxide, carbon dioxide, nitrogen,    methane, ethane, ethene, propane and propene, to obtain a stream    comprising butadiene and n-butane, with or without 1-butene and    2-butene, and with or without oxygenates as further secondary    components;-   (D3) optionally removing the oxygenates to obtain a stream    comprising butadiene and n-butane, with or without 1-butene and    2-butene;-   (D4) separating the stream comprising butadiene and n-butane, with    or without 1-butene and 2-butene, into a stream comprising n-butane,    with or without 1-butene and 2-butene, and a stream comprising    butadiene;-   (D5) optionally recycling the stream comprising n-butane, with or    without 1-butene and 2-butene, into the nonoxidative catalytic    dehydrogenation (B).

After leaving the dehydrogenation stages, the hot gas mixture whosetemperature is generally from 220 to 490° C. is customarily cooled withwater. This condenses out steam and any high-boiling organic secondarycomponents.

The low-boiling secondary components such as hydrogen, carbon monoxide,carbon dioxide, nitrogen, methane, ethane, ethene, propane and propenepresent in the dehyrogenation gas mixture in addition to butadiene,n-butane and any 1-butene and 2-butene are subsequently removed from theC₄ hydrocarbons.

The low-boiling secondary components may be removed by customaryrectification.

The low-boiling secondary components may also be removed in anabsorption/desorption cycle using a high-boiling absorbent. In this way,substantially all low-boiling secondary components (nitrogen, hydrogen,methane, ethane, ethene, propane, propene, carbon oxides, oxygen) areremoved from the n-butane dehydrogenation product gas stream.

To this end, the C₄-hydrocarbons are absorbed in an inert absorbent inan absorption stage to obtain a C₄-hydrocarbon-laden absorbent and anoffgas comprising the remaining secondary components. In a desorptionstage, the C₄-hydrocarbon and traces of secondary components arereleased again from the absorbent.

Inert absorbents used in the absorption stage are generally high-boilingnonpolar solvents in which the hydrocarbon which is to be removed has adistinctly higher solubility than the remaining components of theproduct gas mixture. The absorption may be effected by simply passingthe product gas mixture through the absorbent. However, it may also beeffected in columns or in rotary absorbers. Operation may be effected incocurrent, countercurrent or crosscurrent. Examples of useful absorptioncolumns include tray columns having bubble, centrifugal and/or sievetrays, columns having structured packings, for example sheet metalpackings having a specific surface area of from 100 to 1 000 m²/m³ suchas Mellapak® 250 Y, and randomly packed columns. However, usefulabsorption columns also include trickle and spray towers, graphite blockabsorbers, surface absorbers such as thick-film and thin-film absorbersand also rotary columns, plate scrubbers, cross-space scrubbers androtary scrubbers.

Useful absorbents are comparatively nonpolar organic solvents, forexample aliphatic C₈-to C₁₈-alkenes, or aromatic hydrocarbons such asthe middle oil fractions from paraffin distillation, or ethers havingbulky groups, or mixtures of these solvents, to each of which a polarsolvent such as 1,2-dimethyl phthalate may be added. Further usefulabsorbents include esters of benzoic acid and phthalic acid withstraight-chain C₁-C₈-alkanols, such as n-butyl benzoate, methylbenzoate, ether benzoate, dimethyl phthalate, diethyl phthalate, andalso heat carrier oils, such as biphenyl and diphenyl ether, theirchlorine derivatives and also triarylalkenes. A useful absorbent is amixture of biphenyl and diphenyl ether, preferably in the azeotropiccomposition, for example the commercially available Diphyl®. Frequently,this solvent mixture comprises dimethyl phthalate in an amount of 0.1 to25% by weight. Further useful absorbents are octanes, nonanes, decanes,undecanes, dodecanes, tridecanes, tetradecanes, pentadecanes,hexadecanes, heptadecanes and octadecanes, or fractions obtained fromrefinery streams which have linear alkanes mentioned as main components.

For desorption, the laden absorbent is heated and/or decompressed to alower pressure. Alternatively, desorption may also be effected bystripping or in a combination of decompression, heating and stripping inone or more process steps. The absorbent regenerated in the desorptionstage is recycled into the absorption stage.

A stream consisting substantially of butadiene and n-butane remainswhich may also comprise 1-butene and 2-butene and also oxygenates asfurther secondary components. Such oxygenates include, for example,furan and maleic anhydride. The oxygenates may be removed from the C₄hydrocarbons in a further separating stage which may likewise beconfigured as an absorption/desorption stage or as a rectification.

The remaining stream which customarily consists predominantly ofbutadiene and n-butane and, in addition, may also comprise small amountsof 1-butene and 2-butene may be separated in a further separating stagein a stream comprising n-butane and any 1-butene and 2-butene, and astream comprising butadiene. The separation may be effected, forexample, by butadiene scrubbing. Butadiene scrubbing may be effected asdescribed in Weissermehl/Arpe, Industrielle Organische Chemie, 5^(th)Edition 1998, p. 120/121, or Hydrocarbon Processing, Mar. 2002, p. 50B.

The stream comprising n-butane and any 1-butene and 2-butene may atleast partially be recycled into the nonoxidative catalyticdehydrogenation (B).

Process part (D) preferably comprises at least the steps (D1), (D2) and(D4). More preferably, it comprises the steps (D1) to (D5).

The invention is illustrated hereinbelow with reference to the drawing.

The FIGURE shows the process flow diagram of a preferred embodiment ofthe process according to the invention. A feed stream 1 of liquefiedpetroleum gas (LPG) which consists substantially of propane, n-butaneand isobutane and, in addition, may also comprise methane, ethane orpentanes, is fed to a rectification column 2 and separated into a stream3 composed substantially of propane and any methane and ethane, and astream 4 composed substantially of n-butane and isobutane and anypentanes. In the rectification column 5, any pentanes 6 are removed. Thebutane mixture 7 is separated in the rectification column 8 intoisobutane 9 and n-butane 12, and isobutane is isomerization in theisomerization reactor 10 to an n-butane/isobutane mixture 11 which isfed back into the rectification column 8. n-Butane is fed as the feedgas stream 12 into the first dehydrogenation stage 15 in which anonoxidative catalytic dehydrogenation of butane to 1-butene, 2-buteneand butadiene takes place. This is preferably carried out underautothermal conditions while feeding in oxygen or air as cofeed 13 andoptionally hydrogen as cofeed 14. Preference is given to carrying outthe first dehydrogenation stage with backmixing in a fluidized bed orwith partial gas recycling, for example as described in German patentapplication P 102 11 275.4, unpublished at the Priority date of thepresent invention. The product gas stream 16 leaving the firstdehydrogenation stage which, in addition to butadiene, 1-butene,2-butene and unconverted n-butane, comprises steam and customarysecondary components such as hydrogen, carbon oxides, nitrogen,hydrogen, methane, ethane, ethene, propane and propene is fed to asecond dehydrogenation stage 18, in which while feeding in oxygen or airas cofeed 17, an oxydehydrogenation of 1-butene and 2-butene tobutadiene takes place. The second dehydrogenation stage is preferablycarried out in a tube bundle reactor. The second dehydrogenation stagemay itself be carried out in more than one stage, for example in twostages. In the two-stage configuration of the oxydehydrogenation, thesecond dehydrogenation stage consists of a first oxydehydrogenationstage 18 and a second oxydehydrogenation stage 18 a, into each of whichair or oxygen is fed as cofeed 17 or 17 a. The product gas stream 19 aleaving the second dehydrogenation stage (in the one-stage configurationof the oxydehydrogenation, this is the product gas stream 19) comprises,in addition to butadiene and unconverted n-butane, steam and secondarycomponents such as hydrogen, carbon oxides, nitrogen, methane, ethane,ethene, propane and/or propene, with or without small residues of1-butene and 2-butene and with or without oxygen and oxygen-containinghydrocarbons (oxygenates). The product gas stream 19 a, optionally afterprecooling in heat exchangers, is cooled in the cooling and condensationunit 20 which may be configured, for example, as a water fluidized bedor as a falling-film condenser, to such an extent that water andhigh-boiling organic by-products such as high-boiling hydrocarbons andoxygenates condense out and are discharged from the process as stream21. The uncondensed product gas components are fed to the separatingstage 23 as stream 22 in which a removal of low boilers anduncondensable secondary components 24 (when present in product gasstream 19 a: hydrogen, carbon oxides, nitrogen, methane, ethane, ethene,propane, propene and oxygen) takes place. The separating stage 23 may beconfigured as a rectification column or as an absorption/desorptionunit. The stream 25 comprising the C₄ products of the dehydrogenation,unconverted n-butane and any oxygenates such as furan and maleicanhydride is optionally fed to a further separating stage 26 which maybe configured as a rectification column or an absorption/desorptionunit. In the separating stage 26, oxygenates and any remaining watertraces are removed and discharged from the process as stream 27. Thestream 28 composed of butadiene and n-butane which may also comprisesmall proportions of 1-butene and 2-butene is fed to a furtherseparating stage 29, for example a butadiene scrubbing, and separatedthere into a stream 31 composed of n-butane and any 1-butene and2-butene and a stream 30 composed of butadiene. The stream 31 may atleast partially be recycled into the nonoxidative catalyticdehydrogenation stage 15.

The invention is illustrated by the examples hereinbelow.

EXAMPLES Example 1

Preparation of a Dehydrogenation Catalyst Precursor

A solution of 11.993 g of SnCl₂.2H₂O and 7.886 g of H₂PtCl₆.6H₂O in 600ml of ethanol is poured over 1 000 g of a spalled ZrO₂/SiO₂ mixed oxidehaving a ZrO₂/SiO₂ weight ratio of 95:5 from Norton (USA).

The mixed oxide has the following specifications:

Sieve fraction 1.6 to 2 mm; BET surface area: 86 m²/g; pore volume: 0.28ml/g (from mercury porosimetry measurement).

The supernatant ethanol is taken off on a rotary evaporator using awater jet pump vacuum (20 mbar). Drying is then effected at 100° C. for15 h followed by calcining at 560° C. for 3 h, each under stationaryair. A solution of 7.71 g of CsNO₃, 13.559 g of KNO₃ and 98.33 g ofLa(NO₃)₃.6H₂O in 2 500 ml of H2O is then poured over the dry solid. Thesupernatant water is taken off on a rotary evaporator using a water jetpump vacuum (20 mbar). Drying is then effected at 100° C. for 15 hfollowed by calcining at 560° C. for 3 h, each under stationary air.

The resulting catalyst precursor has a composition ofPt_(0.3)Sn_(0.6)Cs_(0.5)K_(0.5)La_(3.0) (indices represent weightratios) on (ZrO₂)₉₅ (SiO₂)₅ as carrier indices represent weight ratios).

Example 2

Charging of a Dehydrogenation Zone A Reactor and Activation of theCatalyst Precursor

20 ml of the catalyst precursor obtained from example 1 are used tocharge a vertical tubular reactor (reactor length: 800 mm; wallthickness: 2 mm; internal diameter: 20 mm; reactor material: internallyalonized, i.e. aluminum oxide-coated, steel tube; heating: electricalusing an oven from HTM Reetz, LOBA 1100-28-650-2 at a longitudinalmidpoint length of 650 mm). The length of the catalyst bed is 75 mm. Thecatalyst bed is disposed at the longitudinal midpoint of the tubularreactor. The remaining reactor volume above and below is filled withsteatite spheres as an inert material (diameter 4-5 mm) which aresupported from below on the catalyst base.

The reactor tube is then charged at an external wall temperature alongthe heating zone of 500° C. with 9.3 l/h (stp) of hydrogen over 30 min.At the same wall temperature, the hydrogen stream is initially replacedover 30 min by a stream of 80% by volume of nitrogen and 20% by volumeof air at 23 l/h (STP) and then over 30 min by an identical stream ofpure air. While retaining the wall temperature, purging is then effectedwith an identical stream of N₂ over 15 min and finally reduction is onceagain effected with 9.3 l/h (STP) of hydrogen over 30 min. Theactivation of the catalyst precursor is then completed.

Example 3

Preparation of an Oxydehydrogenation Catalyst

1750.9 g of aqueous cobalt nitrate solution having a free HNO₃ contentof 0.2% by weight and a Co content of 12.5% by weight (=3.71 mol of Co)are initially charged in a heatable glass 10 L solid reactor. 626.25 gof solid Fe(NO₃)₃.9H₂O having an Fe content of 14.2% by weight (=1.59mol of Fe) are dissolved with stirring at room temperature in theinitially charged cobalt nitrate solution. 599.5 g of bismuth nitratesolution having a free HNO₃ content of 3% by weight and a Bi content of11.1% by weight (=0.32 mol of Bi) are added to the solution obtained atroom temperature. 106.23 g of solid Cr(NO₃)₃.9H₂O (=0.27 mol of Cr) arethen added. After heating to 60° C. and further stirring, a red solution(solution A) is obtained.

In a heatable 3 l stirred glas vessel, 2 000 ml of water are initiallycharged. 2.38 g of KOH (=0.042 mol of K) and 1 124.86 g of(NH₄)₆Mo₇O₂₄.4H₂O (=6.37 mol of Mo) are then added and are dissolved at60° C. The solution obtained exhibits slight turbidity (solution B).

Solution B is then pumped into solution A while stirring the latter.102.05 g of SiO₂ sol having an SiO₂ content of 50% by weight (“Ludox ™”from DuPont =0.85 mol of Si) are added to the dark yellow suspensionobtained at 60° C.

The suspension obtained is stirred at 60° C. for 30 minutes and thenspray-dried (entrance temperature 370° C., exit temperature 110 to 112°C.). The spray powder obtained is admixed with 4% weight of graphite andthen tableted to solid tablets having a diameter of 5 mm and a height of3 mm. The solid tablets are heat treated at 480° C. for 6 hours in amuffle furnace on a wire sieve (mesh size 3.5 mm) flowed through by airwith air flowing through at a rate of 100 l/h. The calcined tablets arecomminuted through a wire sieve to give catalyst spall having an averagegranulate diameter of from 2 to 3 mm.

The oxydehydrogenation catalyst has the nominal compositionMo₁₂Bi_(0.6)Fe₃Co₇Cr_(0.5)Si_(1.6)K_(0.08)O_(x) (indices representatomic ratios).

Example 4

Charging of a Dehydrogenation Zone B Reactor

95 ml of catalyst precursor obtained for example 3 are used to charge avertical tubular reactor (reactor length: 100 cm; wall thickness: 2 mm;internal diameter: 13 mm, reactor material: internally alonized steeltube with a thermowell disposed therein having an external diameter of 2mm which contains a moveable thermal element; heating: electrical withthree different heating zones over the reactor length of 100 cm usingheating collars from Winkler, Heidelberg, and a maximum isothermallength of 82 cm is achieved over the middle region of the reactor). Thelength of the catalyst bed is 82 cm. The catalyst bed is in theisothermal region of the tubular reactor. The remaining reactor volumeabove and below is charged with steatite spheres as an inert material(diameter 2-3 mm), and the entire reactor tube charge is supported frombelow on a catalyst base of height 5 cm.

Example 5

Dehydrogenation of n-Butane in the Dehydrogenation Zone A Reactor

The dehydrogenation zone A reactor for example 2 is charged at anexternal wall temperature along the heating zone of 500° C. with amixture of 20 l/h (stp) of n-butane, 3.5 l/h (stp) of air, 1.4 l/h (stp)of hydrogen and 10 l/h (stp) of steam as the reaction gas mixture.

The n-butane, air and hydrogen are metered by means of a mass flowregulator from Brooks, while the water is initially metered into anevaporator in liquid form by means of a Kontron HPLC pump 420,evaporated in it and then mixed with the n-butane and the air. Thetemperature of the charge gas mixture in the charge is 150° C. By meansof an REKO pressure regulator at the reactor exit, the exit pressure ofthe tubular reactor is set to 1.5 bar.

An analytical amount of the product gas mixture A obtained isdecompressed to atmospheric pressure and cooled to condense out thesteam present. The remaining gas is analyzed by means of GC (HP 6890with Chem.-Station, detectors: FID; TCD; separating columns: Al₂O₃/KCI(Chrompack), Carboxen 1010 (Supelco)). In a corresponding manner, thecharging gas mixture is also analyzed.

After an operating time of three days, the analysis results reported intable 1 were obtained: TABLE 1 Charging gas mixture [% Product gasmixture by volume] [% by volume] Methane 0.07 Ethane 0.05 Ethene <0.01Propane 0.10 Propene 0.05 H₂ 4.0 16.3 O₂ 2.0 <0.01 N₂ 8.0 6.8 CO 0.03CO₂ 0.28 Isobutane 0.11 n-Butane 57.4 33.2 trans-butene 5.7 cis-Butene4.8 Isobutene 0.08 1-Butene 4.1 Butadiene 0.52 H₂O 28.6 27.7

An n-butane conversion of 32 mol % based on single pass and aselectivity of n-butene formation of 94 mol % corresponds to thesevalues. The selectivity of the butadiene formation corresponds to 3.3%.

Example 6

Dehydrogenation of n-Butane in the Dehydrogenation Zone A Reactor andSubsequent Oxydehydrogenation in the Dehydrogenation Zone B Reactor.

The dehydrogenation zone B reactor from example 4 is heated to atemperature at which the n-butene conversion on single throughput of thereaction gas mixture is >99 mol %, and the internal temperature of thereactor is controlled by means of the thermal elements disposed in theinternal thermowell.

The charge consists of a mixture of 150 l/h (stp) of air (=20° C.) andthe 34.4 l/h (stp) of product gas mixture A from example 5 (=500° C.).The air is metered in by means of a mass flow regulator from Brooks. Thetemperature of the charging gas mixture is brought to the reactorexternal wall temperature. By means of a pressure regulator at thereaction tube exit, the exit pressure of the reactor is set to 1.3 bar.

Downstream of the pressure regulator, the product gas mixture B obtained(temperature =330° C.) is decompressed to atmospheric pressure andanalyzed by means of GC (HP 6890 with Chem-Station; detectors: TCD; FID;separating column: Poraplot Q (Chrompack), Carboxen 1010 (Supelco)). Thecharging gas mixture is analyzed in an identical manner.

After an operating time of 3 days, the results reported in table 2 areobtained. TABLE 2 Charging gas mixture Product gas mixture [% by volume][% by volume] Methane 0.02 0.01 Ethane 0.01 0.01 Ethene <0.01 <0.01Propane 0.02 0.02 Propene 0.01 <0.01 H₂ 3.5 3.5 O₂ 15.7 11.1 N₂ 64.363.5 CO 0.01 1.3 CO₂ 0.06 1.3 Isobutane 0.02 0.02 n-Butane 7.1 7.0trans-Butene 1.2 <0.01 cis-Butene 1.0 <0.01 Isobutene 0.02 <0.011-Butene 0.9 <0.01 Butadiene 0.11 2.6 H₂O 6.0 9.6

An n-butene conversion of 99 mol % based on single pass and aselectivity of butadiene formation of 80 mol % corresponds to thesevalues.

The overall yield of butadiene over both dehydrogenation zones A and Bbased on n-butane used is 25%.

4. Comparative Example

The charging gas mixture described in example 5 is passed directly intothe dehydrogenation zone B reactor. Under identical reaction conditions,there is no conversion of the n-butane to butenes or butadiene.

1. A process for preparing butadiene from n-butane comprising the steps of (A) providing an n-butane-containing feed gas stream, (B) feeding the n-butane-containing feed gas stream into a first dehydrogenation zone and nonoxidatively catalytically dehydrogenating n-butane to 1-butene, 2-butene and optionally butadiene to obtain a first product gas stream comprising n-butane, 1-butene and 2-butene, with or without butadiene and secondary components, (C) feeding the first product gas stream comprising n-butane, 1-butene and 2-butene, with or without butadiene and secondary components, into a second dehydrogenation zone and oxidatively dehydrogenating 1-butene and 2-butene to butadiene to give a second product gas stream comprising butadiene, n-butane and steam, with or without secondary components, (D) recovering butadiene from the second product gas stream.
 2. The process as claimed in claim 1, wherein the provision of the n-butane-containing feed gas stream comprises the steps of (A1) providing a liquefied petroleum gas (LPG) stream, (A2) removing propane and optionally methane, ethane and pentanes from the LPG stream to obtain a butane-containing stream, (A3) removing isobutane from the butane-containing stream to obtain the n-butane-containing feed gas stream and optionally isomerizing the removed isobutane to an n-butane/isobutane mixture and recycling the n-butane/isobutane mixture into the isobutane removal.
 3. The process as claimed in claim 1 or 2, wherein the nonoxidative catalytic dehydrogenation (B) of n-butane is carried out as an autothermal catalytic dehydrogenation.
 4. The process as claimed in any of claims 1 to 3, wherein the oxidative dehydrogenation (C) is carried out in more than one stage.
 5. The process as claimed in any of claims 1 to 4, wherein the recovery (D) of butadiene from the second product gas stream comprises the steps: (D1) cooling the product gas stream with water to condense out steam and any high-boiling organic secondary components; (D2) removing the low-boiling secondary components contained in the second product gas stream which are selected from the group consisting of hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane, ethane, ethene, propane and propene, to obtain a stream comprising butadiene and n-butane, with or without 1-butene and 2-butene, and with or without oxygenates as further secondary components; (D3) optionally removing the oxygenates to obtain a stream comprising butadiene and n-butane, with or without 1-butene and 2-butene; (D4) separating the stream comprising butadiene and n-butane, with or without 1-butene and 2-butene, into a stream comprising n-butane, with or without 1-butene and 2-butene, and a stream comprising butadiene; (D5) optionally recycling the stream comprising n-butane, with or without 1-butene and 2-butene, into the nonoxidative catalytic dehydrogenation (B). 